The expression “syngas” as used herein refers to synthesis gas, which is a common term to refer to gas mixtures comprising carbon monoxide and hydrogen.
The present invention specifically relates to a process for the preparation of syngas using an autothermal reformer (ATR). Such processes are well known in the art. Typically a feed gas comprising methane is contacted with an oxidizing gas in an ATR and the methane reacts with the oxidizing gas to form a raw syngas in an exothermic partial oxidation reaction. The heat generated in this process is largely contained in the hot raw syngas produced. For an optimum heat efficiency of the overall process the heat contained in the hot raw syngas can be recovered for use in the process by indirect heat exchange against water to produce steam (e.g. in a waste heat boiler) and/or by heat exchange against other process streams. For example, the use of so called feed/effluent heat exchangers is desirable, as this allows an effective heat integration. In such feed/effluent heat exchanger the hot syngas produced, typically after being passed through a waste heat boiler, is cooled against the methane-comprising gas feed from which the syngas is produced.
A problem generally encountered with recovering heat from the hot raw syngas by heat exchange, e.g. in a waste heat boiler and/or feed/effluent heat exchanger, is the corrosive nature of the raw syngas. The syngas side of the heat exchanger used is consequently prone to metal dusting corrosion. This can cause serious damage to the heat exchanger. Furthermore, the alloys used for such heat exchangers typically comprise metals such as iron, nickel and/or cobalt. The metal dust produced as a result of metal dusting corrosion, will accordingly comprise these metals which are known to catalyse the methanation reaction. Methanation is the formation of methane from hydrogen and carbon oxides and is an undesired side reaction in the production of syngas, not only because it reduces the amount of hydrogen and carbon monoxide in the syngas, but also because methanation is a strongly exothermic reaction and may cause local damage or failure of the syngas side of the heat exchanger if too much heat is accumulated.
One way to reduce metal dusting in a a reforming process using an ATR could be the addition of a small amount of sulphur to the effluent of feed of the ATR such as disclosed in US20040063797. The process disclosed in US20040063797 is a process in which synthesis gas is produced by combining endothermic adiabatic steam reforming with an ATR. The heat required for the endothermic adiabatic steam reforming step may (partly) be provided by the hot effluent from the ATR. The feed to the ATR, however, should still be heated separately to provide sufficient heat to effectively carry out the oxidation reaction in the ATR.
The addition of sulphur to the effluent of secondary reformer (essentially an ATR) prevent metal dusting is also described in WO2000009441, which discloses a similar process as described in US20040063797. The process as disclosed in WO2000009441 involves an endothermic primary reforming step followed by a secondary partial oxidation step. The primary reforming step takes place in a heat exchange reformer, in which the hot effluent from the secondary reformer transfers its heat to the primary reforming feed gas in the heat exchange reformer. Also in this process the feed to the secondary reformer (equivalent to the ATR) needs to be separately heated.
Another solution to prevent metal dusting may be to apply a coating to the metal surface inside the heat exchanger which is in direct contact with the hot raw syngas. Such coating is, for example, disclosed in WO2010009718. Alternatively, metal alloys specifically developed to withstand aggressive conditions may be used. However, applying a coating or using a specific metal alloy are expensive measures which will add substantially to the cost of the heat exchanger to be used, whilst their effect under the aggressive conditions caused by the hot raw syngas may still be limited.
The present invention aims to provide a more energy efficient syngas manufacturing process which at the same time effectively deals with problems of metal dusting and methanation. More specifically, the present invention aims to make heating of the ATR feed stream more effective, thereby increasing the heat efficiency of the overall syngas producing process, whilst at the same time preventing corrosion of the equipment used.
The present invention thus aims to achieve optimum heat integration, effective prevention of metal dusting corrosion and effective syngas production, both from a yield perspective and from a cost perspective.